The separation of condensable gases, such as carbon dioxide, from other light gases is an important part of gas preparation and purification. For example, the flue gases of a conventional power station typically contain from about 4% (by volume) to about 16% carbon dioxide (CO2). CO2 may represent a significant contributor to climate change. Therefore, there is a clear need for efficient methods of capturing CO2 from flue gases so as to produce a concentrated stream of CO2 that may readily be transported to a safe storage site or to a further application. Similarly, CO2 must be separated from natural gas, syngas and producer gas from gasification or reforming, breathable air, and air inlet for air separation units. Other condensable gases such as water must be separated from air, chemical streams, natural gas, and most of the streams cited earlier. Other condensable gases that must be separated from process streams include natural gas liquids, pollutants such as NOx, SOx, Hg, O3, impurities, contaminants, pollutants, odorous gases, and air toxics. The immediate interest of the inventors is CO2, but the disclosure described here has obvious applications to other gas separation systems.
CO2 has been or is planned to be captured from gas streams by five main technologies: absorption, where CO2 is selectively absorbed into liquid solvents; adsorption or chemical looping, where CO2 is separated by adsorption or reaction on the surfaces of specially designed solid particles which may or may not be induced to release the CO2 into substantially pure streams later in the process; membranes, where CO2 is separated by semi-permeable plastics or ceramic membranes; oxyfiring, where oxygen is separated from air prior to combustion, producing a substantially pure CO2 effluent; and, low temperature/high pressure processes, where the separation is achieved by condensing the CO2.
The currently most well-established and therefore economical proven technique to capture CO2 from a flue gas is to scrub the flue gas with an amine solution to absorb CO2 to the solution. This technology has reached the commercial state of operation for CO2 capture systems from small scale flue gases and from specialty processes. However, its application decreases considerably the total efficiency of the power plant.
Another type of process that has received significant attention is the oxy-combustion systems, which uses oxygen, usually produced in an air separation unit (ASU) but sometimes in membrane separation units, instead of air, for the combustion of the primary fuel. The oxygen is often mixed with an inert gas, such as recirculated flue gas, to keep the combustion temperature and heat absorption at a suitable level. Oxy-combustion processes produce flue gas having CO2, water and O2 as its main constituents; the CO2 concentration being typically greater than about 70% by volume. Treatment of the flue gas is often needed to remove air pollutants and non-condensed gases (such as nitrogen) from the flue gas before the CO2 is sent to storage.
Additionally, a fluidized bed typically has a cooling surface and particles located inside of the fluidized bed and bubbling gas